The present invention has as its object improved powdered pyrotechnic materials and a process for the preparation of such substances. More particularly, the invention has as its object powdered pyrotechnic materials which are less sensitive to external influences, such as mechanical, electric, thermal etc. and a process for their preparation.
Pyrotechnic materials are understood to include pure powdered explosive chemical compounds and pyrotechnic compositions. A pyrotechnic composition is a composition capable of reacting exothermically under the effect of a pyrotechnic or non-pyrotechnic phenomenon to produce a specific effect such as flame, light or sound, evolution of smoke or gas, etc. The pyrotechnic compositions of the invention, more particularly, but not exclusively are priming, conducting, retarding, gas-generating, illuminating, or smoke-producing.
The compositions comprise firstly primary or primary and secondary explosives and secondly, optionally, conducting, oxidizing or reducing additives.
The pure powdered explosive chemical compounds most particularly concerned are primary explosives, that is explosives which are easily detonated by a slight external force which can be mechanical, electrical or thermal etc.
The currently available primary explosives have a very high sensitivity, much higher than necessary. This is a major inconvenience, in certain cases, this excessive sensitivity prevents any handling for charging pyrotechnic devices and makes the correct utilization of these devices very delicate.
In addition to their excessive sensitivity the primary explosives now in use have the disadvantages inherent in powdered products. Their low variable density does not favor any satisfactory volumetric charging of the devices, the flowability is mediocre; these powdered products contain "fines" (that is, small particles) which have the tendency to disperse easily in the atmosphere and to form dangerous dusts; moreover, the reproducibility of pyrotechnic characteristics from one preparation to another is very difficult to obtain.
Attempts have been made in the prior art to reduce the sensitivity of primary explosives; they consist, notably, in introducing during the synthesis of the explosive, such as lead nitride, inert substances such as dextrin, the sodium salt of carboxymethylcellulose, polyvinyl alcohol etc. This addition has the serious disadvantage of producing a decrease in performance and to render more difficult the reproducibility of the pyrotechnic characteristics from one preparation to another.
It is also known in this art to reduce the sensitivity of powdered substances and to improve their pyrotechnic characteristics by coating the powder particles with a polymeric binder. This coating method consists of mixing the explosive particles with a solution of a polymer and evaporating the solvents. By this method are obtained non-homogeneous products of a very mediocre quality; there is obtained a more or less compact mass which is then subjected to suitable mechanical processes to effect granulation. This process is not satisfactory because there is obtained a mixture of coated particles having on their surface free crystals of the explosive and/or incompletely coated particles.
U.S. Pat. No. 3,646,174 describes a process for obtaining spheroidal particles coated with an organic polymer. This process comprises the steps of mixing solid particles with a liquid organic prepolymer which can be transformed into a solid polymer, and a volatile liquid which is non-miscible with the prepolymer, of agitating the resultant mixture and simultaneously evaporating the volatile liquid to form spheroidal globules of liquid prepolymer containing the particles; the last step consists of continuing the agitation and removal of the volatile liquid until the prepolymer is transformed into a solid polymer to form spheroidal agglomerates of said solid particles in an organic polymer matrix. This patent is illustrated by examples in which it is specified that 95 percent of the particles having a particle size of 2 to 3 microns are coated, the dimension of the spheroids being between 0.3 and 1 mm.; this value of 5 percent of non-coated particles cannot be tolerated in coating primary explosives, if it is desired to control their sensitivity effectively.
In the case of a primary explosive, the presence of a single non-coated or non-uniformly coated particle cannot be tolerated; in effect the sensitivity of the explosive would then be reduced to the intrinsic sensitivity of the basic explosive and the coating process would be rendered useless.
The present invention overcomes the above-mentioned disadvantages.